Power supply distributed load startup system

ABSTRACT

A power supply system (300) provides a staged startup of multiple loads (303, 305). Preferably, the power supply (300) is a switching power supply that provides power, in the form of a primary output voltage to a first output terminal (319) and to a second output terminal (325). A first load (303) demands a first startup power from the switching power supply through the first output terminal (319). A voltage comparator (333) provides an enable signal (335) when a primary output signal, present at the first output terminal, exceeds a predetermined threshold (335). A load coupling device, preferably a gateable voltage regulator (331), is coupled to the second output terminal (325). The gateable voltage regulator (331) has an output (327) coupled to a second load (305). The gateable voltage regulator (331), responsive to the enable signal (335), provides a coupling between the second output terminal (325) and the second load (305), whereby the second load (305) demands a second startup power from the power supply through the second output terminal (325).

FIELD OF THE INVENTION

This invention is generally directed to the field of power supplies, andspecifically to the aspect of reduction of power required at powersupply startup.

BACKGROUND OF THE INVENTION

Power supply systems must not only provide operative power to loadsduring normal operating conditions, but also during startup orinitialization of power to the loads. The demand for smaller productsize has caused an evolution from relatively simple linear powersupplies to more complex switching power supplies. FIG. 1 illustrates apower supply 101. This power supply 101 has multiple outputs connectedto multiple loads 103, 105, and 107. Each of these loads have both astartup power requirement and an operating power requirement. Thesepower requirements are illustrated in FIG. 2.

In FIG. 2 various waveforms associated with the startup power andoperating power requirement for each of the loads 103, 105, and 107 isshown. For simplicity, a single waveform V_(OUT) 201 shows a voltageover time representative of each of several voltages driving the loads103, 105, and 107. Note that the voltage slowly rises 203 from zero to astable voltage 205. During the transition 203 the loads 103, 105, and107 are demanding the aforementioned startup power. When the voltagestabilizes, as shown at reference number 205, the loads 103, 105, and107 are demanding operative power.

I_(LOAD1) 207, I_(LOAD2) 213, and I_(LOAD"n") 219, represent loadcurrents demanded by each of the loads 103, 105, and 107. I_(COMPOSITE)225 represents the combined, or composite load current demanded from thepower supply 101. A startup portion of each of the current waveforms isrepresented by reference numbers 209, 215, 221, and 227. An operatingportion of each of the current waveforms is represented by referencenumbers 211, 217, 223, and 229. The power demanded from the power supply101 is the product of the voltage, represented simply by V_(OUT) 201,and the composite current I_(COMPOSITE) 225. Computation will show afairly high startup power requirement for this power supply because ofthe fairly high startup current requirements of the individual loads103, 105, and 107. High startup current can be partially attributable tocross-conduction in CMOS (complementary metal oxide semiconductor)integrated circuits, and other loads that are operating in anindeterminate state until their control circuits are powered-up.

Physical size, and stress, thus field reliability, of a power supply isdependent on this startup power requirement. This is true for simplelinear, and more complex switching type power supplies. Inherently,switching power supplies have more internal components that are effectedby both the startup power requirement and the operating powerrequirement. Since switching power supplies inherently switch on and offcurrent through certain reactive components, these relatively highstartup power demands especially tax certain components. Particularly,capacitors have a ripple current rating associated with a capacitor'sability to thermally recover from a transient change in current throughthe capacitor. As transient power demand increases linearly, thecapacitor increases volumetrically to safely provide this power demand.Also certain inductors, indigenous to switching power supplies, mustalso increase volumetrically so that the core elements are not saturatedduring these relatively high power demands during startup.

Also, active switching elements, typically a FET (field effecttransistors) or other type of semiconductor switch, need to havesufficient bulk to handle the high startup power demand. In any case,the FET will need to operate at a higher temperature to account for thisincreased power demand during startup.

Further, prior art switching power supplies require that theabove-mentioned components have to be over-sized because of a minimumstartup voltage problem inherent in these designs. Essentially, a sourcevoltage for the switching power supply must be sufficiently high for theregulator to startup and operate properly. This becomes problematic whenthe switching power supply has a fairly high startup current demand anda current limiting circuit that limits power dissipation in theswitching power supply. When the source voltage is applied to theswitching power supply, the output voltage of the switching power supplystarts to build. Responsive to this building voltage, the connectedloads start to draw current. If the current demand exceeds the currentlimit, then the output voltage will be caused to reduce to a levelcoincident with a predetermined maximum power dissipation. This meansthat if the current limit isn't set high enough then the switching powersupply's output voltage will never reach the specified voltage--thus notstart up correctly. To prevent this from happening, the current limitneeds to be set high enough to support a low source voltage startupsequence under the maximum startup loading condition. Setting thecurrent limit higher requires volumetrically larger capacitors,inductors and a larger FET. Significantly, setting the current limithigher to accommodate this high startup current demand, also increasesthe headroom necessary to accommodate short circuit protection. Forinstance, if operating current is 5 amps, and startup current is 10amps, then short circuit test limit must be above 10 amps. Theadditional headroom associated with this short circuit test limit willcause the aforementioned components to grow even larger and dissipatemore power--thus heat.

As mentioned earlier, the switching power supply must also dissipatethis extra power associated with the short circuit test limit, under alow source voltage condition during startup. Typically, the startupcurrent associated with a switching power supply may be 200-300% of themaximum operating current. As mentioned earlier, this causes theinductors and capacitors to grow substantially in size. Also, the activeswitching element needs to be significantly oversized to survive thissubstantial startup current.

What is needed is an improved power supply system that is relativelycompact, reliable, easily manufacturable, and can efficiently managethese relatively high startup power demands.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system block diagram illustrating a general configuration ofa power supply driving multiple loads as known in the prior art; and

FIG. 2 is a chart of waveforms associated with operation of the priorart system shown in FIG. 1;

FIG. 3 is a general system block diagram in accordance with theinvention;

FIG. 4 is a chart of waveforms associated with operation of the systemillustrated in FIG. 3; and

FIG. 5 is a detailed system block diagram in accordance with theinvention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

A system provides a distributed load startup apparatus that minimizesstartup power required from a power supply, by staging the startup ofmultiple loads. Because of this minimization of startup power, certainkey components may be downsized, saving space and cost, and the powersupply can operate more reliably because thermal dissipation can beminimized.

Preferably, the power supply is a switching power supply that provides aprimary power, in the form of an output voltage, to a first outputterminal and to a second output terminal. A first load demands a firststartup power from the switching power supply through the first outputterminal. A voltage comparator provides an enable signal when theprimary output voltage, present at the first output terminal, exceeds apredetermined voltage threshold. Alternatively, a signal representativeof power or current demanded may be compared to a threshold to generatethe enable signal. A load coupling device, preferably a gateable voltageregulator, is coupled to the second output terminal. The gateablevoltage regulator has an output coupled to a second load. The gateablevoltage regulator, responsive to the enable signal, provides a couplingbetween the second output terminal and the second load, whereby thesecond load demands a second startup power from the power supply throughthe second output terminal. This approach ensures that the startup loadpower demanded from the first load stabilizes to a lower operating powerbefore the second load is connected and demanding startup power. Becauseof this, the composite startup power is less than the summation of bothload's startup power demand. Of course, this approach may be extended toadd more staged loads by slightly modifying this same apparatus.

FIGS. 1 and 2 were introduced in the background section of thisapplication and essentially describe the high startup power demandproblem existing in the prior art.

FIG. 3 is a system block diagram illustrating a general configuration ofthe improved power supply, or distributed load startup apparatus.Preferably, this improved power supply is installed in a vehicle poweredby a battery 301. Alternatively, many other applications andconfigurations can benefit from the described system. The battery 301provides power to the power supply which ultimately provides power tovarious loads, here represented by reference numbers 303 LOAD1 and 305LOAD2. The battery 301, or other power source, is coupled to the powersupply, in this case a switching regulator control circuit 307. Theswitching regulator control circuit 307, is coupled to and driving aprimary winding 309 of a transformer 311. A primary output voltage isprovided at a terminal 313.

The transformer 311 has at least a first secondary winding 315, and asecondary winding 317. The primary output voltage, provided at aterminal 313, is coupled via the transformer 311 to the secondarywindings 315 and 317. A first, or first secondary, output terminal 319,provides a first voltage that is coupled to a terminal (V_(LOAD1)) 323through a rectifier-filter device 321 to the first load, LOAD1 303. Asecond, or second secondary, output terminal 325, provides a secondvoltage that is coupled to a terminal (V_(LOAD2)) 327 through arectifier-filter device 329 and a gateable regulator 331 having anoutput 332 to the second load, LOAD2 305.

A comparison means, in this case a voltage comparator 333, is coupled toa threshold element 334 that provides a threshold voltage, and also tothe (V_(LOAD1)) terminal 323, which provides a voltage representative ofthe first voltage provided by the first secondary output terminal 319.

Note that elements 307, 309, 315, 321, and the feedback path shown byreference number 339 in combination form a voltage regulator. Althoughin the preferred embodiment the aforementioned gateable regulator 331has regulation capabilities this is not necessary for operation of theimproved system. However, in typical power supply systems this deviceincludes regulation because it doesn't benefit from the regulationaction caused by the feedback signal associated with the line shown byreference number 339.

Now that some of the essential elements have been introduced, theintroduction of the remaining elements and the operation of the circuitin FIG. 3 will be described concurrently. Both FIG. 3 and FIG. 4 will bereferenced in this next description. FIG. 4 illustrates variouswaveforms that illustrate the operation of the circuit in FIG. 3.

When power is applied to the switching regulator control circuit 307from the battery 301, the switching regulator control circuit 307 willcommence operation. This means that the switching regulator controlcircuit 307 will cause the primary output voltage, provided at aterminal 313 to switch on and off at a relatively high frequency thusproviding a primary output voltage at terminal 313. Since the primaryoutput voltage is coupled, via the transformer 311, to the firstsecondary winding 315, the voltage at the first secondary outputterminal 319 will respond to this primary output voltage by increasing.This will, through the action of the rectifier-filter element 321, causethe voltage V_(LOAD1) 323 to increase. This increase in voltageV_(LOAD1) 323 is shown in waveform 401 of FIG. 4 at reference number403. While this voltage V_(LOAD1) 323 is increasing, the first loadLOAD1 303 starts demanding a primary startup power from the switchingregulator control circuit 307, through the first secondary outputterminal 319. This is startup power is dependent on the demanded loadcurrent which is shown in waveform I_(LOAD1) 416. Until the load, thusthe load current demand, stabilizes, its power demand thus the loadcurrent demanded from the switching regulator control circuit 307, willbehave in a transient manner as shown by reference number 417. Asmentioned in the background section, this transient load startupcurrent--thus power may be partially attributable to cross-conduction inCMOS (complementary metal oxide semiconductor) integrated circuits, orinitial charging of capacitors associated with LOAD1 303, and otherloads that are operating in an indeterminate state until theirrespective control circuits are powered-up.

Another waveform, I_(COMPOSITE) 420 illustrates, at reference number421, this transient load startup current 417--thus power demanded fromthe switching regulator control circuit 307. Since the second load isnot activated at this time, the transient load startup current 417 shownin the composite waveform I_(COMPOSITE) 420 is much smaller than in theprior art circuit. This is because a demand from the second load is notpresent.

When the voltage V_(LOAD1) 323 increases above the threshold voltage,shown in FIG. 4 at reference number 405 and provided by the thresholdelement 334, an enable signal 335 will be generated. The first load'spower demand stabilizes when the voltage V_(LOAD1) 323 stabilizes, asshown by reference number 406, shortly thereafter. As a result thetransient startup load current is reduced to a steady-state operatingcurrent. This is shown in waveforms V_(LOAD1) 401, I_(LOAD1) 416, atreference number 419, and I_(COMPOSITE) 420, at reference number 423.The threshold voltage is indicative of this stabilization.

The enable signal 335 is illustrated graphically in FIG. 4 in referencewaveform 407, and is shown activated at reference number 409 responsiveto the comparator's 333 action. This enable signal 335 is coupled to acontrol input 337 on the gateable regulator 331. When the enable signal335 is provided, The gateable regulator 331 causes a coupling betweenthe second output terminal 325 and the output 332 and the second load305. Prior to this action caused by the enable signal, no voltage, wasapplied to the second load 305 or at the output 332. Thus, the secondload 305 did not demand any current, thus power. As a result of thecoupling, the second load 305 demands startup power from the switchingregulator control circuit 307 through the second output terminal 325.The result of this demanded startup power is a startup load currentshown in waveform I_(LOAD2) 424 at reference number 425, and also in theI_(COMPOSITE) 420 waveform at reference number 429. The demanded startupload power is the product of the load current, shown in waveform 424 andthe increasing load voltage V_(LOAD1) 327 shown in waveform 411 of FIG.4 at reference number 413.

Of course, other means, such as a measurement of the stabilization ofstartup current or startup power demand may also be applied to determinethe stabilization of the startup current of the first load and therebythe enablement of the second load.

Note that because LOAD1 303 is already stable, it may, via a controlcoupling 341, provide a stable control signal to the second load 305.This control coupling represents, for instance, a microprocessor'soutput control signals, associated with LOAD1 303, controlling an inputto a solenoid circuit of LOAD2 305. By doing so, the second load 305 isable to startup more gracefully, thus demanding less current during itsstartup phase. This is because the control input deciding the state ofthe current demanding elements, indigenous to LOAD2 305 can now startupin a stable state.

The second load's power demand stabilizes when the voltage V_(LOAD2) 327stabilizes, as shown by reference number 415, shortly thereafter. As aresult the startup load current is reduced to a steady-state operatingcurrent. This is shown in waveforms V_(LOAD2) 411, I_(LOAD2) 424, atreference number 427, and reflected in a new I_(COMPOSITE) 420, atreference number 431. At this time I_(COMPOSITE) 420 now represents boththe operating current attributable to the I_(LOAD1) demand and theI_(LOAD2) demand.

Because the startup of these multiple loads 303 and 305 is staged,responsive to the stabilization of the previously starting load, thetotal instantaneous current, thus power demanded from the power means,in this case the switching regulator control circuit 307, is reducedcompared to prior art circuits. This allows capacitors, inductors,resistors, and transistors in the switching regulator control circuit307 and the rectifier-filters 321, 329, to be sized significantlysmaller than in prior art designs. These elements will be illustratednext in FIG. 5. This also reduces the power requirement for thetransformer 311.

FIG. 5 is a system block diagram showing additional details of thecircuit shown in FIG. 3. Capacitors 501, 503, 505, 507, 509, and 511 maybe sized substantially smaller than prior art designs because ripplecurrent, associated with the demanded load current, can be reducedsignificantly. Inductors 513, 515, 517 and the transformer 511, can alsobe sized substantially smaller than prior art designs because the demandfor load current can be reduced significantly. Similarly, the FET 519may also be reduced in size and be asked to dissipate less heat. All ofthese improvements make the system more manufacturable, more compact,less costly, and more reliable.

As mentioned above, this approach may be extended to add more stagedloads by slightly modifying this same apparatus. An example of this isshown in FIG. 5 with V_(LOAD"n") 521 and switched regulator 522 whichcan be switched either directly based on the output of comparator 333 orindirectly based on the output of an additional comparator whichmonitors the output 332.

In conclusion, this system provides a distributed load startup apparatusthat minimizes instantaneous startup power required from a power supply.Because of this minimization of startup power, certain key componentsmay be downsized and the power supply can operate more reliably becausethermal dissipation can be minimized. Also, the minimum startup voltageproblem, inherent in prior art designs, greatly diminished in this casebecause the high startup load current demands that cause the problemhave been reduced by the staged turn-on. Additionally, the headroomassociated with a short circuit test limit need not grow the loadcurrent demand beyond a point manageable by the key components.

What is claimed is:
 1. A power supply for providing power to at least afirst and second load, the power supply comprising:a power supply forproviding voltage to the first load via a terminal; a comparator coupledto the terminal for providing a signal when the voltage provided to thefirst load exceeds a fixed threshold; and a voltage regulator coupledbetween the power supply and the second load, the voltage regulatorproviding operative power to the second load responsive to the providedsignal, wherein magnitudes of currents flowing through the first loadand the second load are different.
 2. A switching power supplydistributed load startup system having a primary load and a secondaryload, said system comprising:a switching power supply having a primarycircuit output for providing power; a first secondary output circuitcoupled to the primary circuit output of said switching power supply forproviding the power to said primary load through a first outputterminal; a second secondary output circuit coupled to the primarycircuit output of said switching power supply for providing the power tosaid secondary load through a second output terminal; a voltagecomparator coupled to the first output terminal of said first secondaryoutput circuit, said voltage comparator for providing an enable signalresponsive to a voltage present at the first output terminal exceeding apredetermined voltage threshold; and a gateable voltage regulatorconnected to the second output terminal of said second secondary outputcircuit, said gateable voltage regulator including an output coupled tothe secondary load, said gateable voltage regulator being responsive tothe enable signal to provide a coupling between the second outputterminal and the second load.
 3. A switching power supply in accordancewith claim 2 wherein the primary circuit output comprises a primarywinding of a transformer and each of the first and second secondaryoutput circuit's comprise secondary windings coupled to the primarywinding of the transformer.
 4. A power supply for providing current toat least a first and second load, the power supply comprising:a powersupply for providing operative current to the first load via a terminal;and a regulator coupled to the power supply, the regulator for providingoperative current to the second load, while a signal measured at theterminal exceeds a predetermined threshold.